Ring-pairing method

All these methods demonstrate that the 2-positions of pyridine, pyrimidine, and other azines are the most electron deficient in the ground state. However, considerably greater chemical reactivity toward nucleophiles at the 4-position is often observed in syntheses and is supported by kinetic studies. Electron deficiency in the ground state is related to the ability to stabilize the pair of electrons donated by the nucleophile in the transition state. However, it is not so directly related that it can explain the relative reactivity at different ring-positions. Certain factors which appear to affect positional selectivity are discussed in Section II, B. 

The basis set is 6-31G(d,p), and electron correlation at the MP2 level is included. A similar structure is obtained with the AMI and PM3 semi-empirical methods. Density functional theory at the B3LYP/6-31G(dp,p) level also produced the same structure for this ion-pair. The only observed differences between the semi-empiri-cal and the ab initio structures were slightly shorter hydrogen bonds (PM3 and AMI) between FI, F2, and F5 and the G2-F1 (H18) on the imidazolium ring. 

The longer the chain of unbranched carbenium ions is, the more the calculated values deviate from those found experimentally in the direction of higher stability. However, the expected order of ion stability (primary < secondary < tertiary) remains intact. For cations, which are able to delocalize the positive charge due to conjugation in phenyl rings, the calculated stability is too small. The example of the acetyl cation shows that the reliability of the MINDO/3 method decreases, if charged species, especially those containing hetero atoms with free electron pairs, are calculated. 

Rings with more than two differently substituted carbons can be dealt with on similar principles. In some cases, it is not easy to tell the number of isomers by inspection. The best method for the student is to count the number n of differently substituted carbons (these will usually be asymmetric, but not always, e.g., in 68) and then to draw 2" structures, crossing out those that can be superimposed on others (usually the easiest method is to look for a plane of symmetry). By this means, it can be determined that for 1,2,3-cyclohexanetriol there are two meso compounds and a dl pair and for 1,2,3,4,5,6-hexachlorocyclohexane there are seven meso compounds and a dl pair. The drawing of these structures is left as an exercise for the student. 

The lack of stereospecificity in equation (72) (as compared to equations (70) and (71)) is readily accommodated by the long lifetime of the triplet ion pair produced by the sensitized irradiation method. The latter allows the facile ring opening of DBC+ cation radical followed by isomerization of the resulting xylylene cation radical, i.e.,... 

---